1. Field of the Invention
The present invention relates to an electron gun assembly for a color cathode ray tube, and more particularly to an electron gun assembly capable of improving its focus characteristics and preventing a moirxc3xa9 phenomenon on the overall region of the screen, thereby realizing a high quality image.
2. Description of the Related Art
In general, a cathode ray tube is a display device which is used in the fields of a television receiver; an oscilloscope, a radar viewing device, etc. and which displays an image on the front surface of a panel by controlling electron beams emitted from the electron gun assembly so as to impinge phosphors deposited on an inner surface of the panel of the tube, depending on received image signals.
As a construction of such a color cathode ray tube, as shown in FIG. 1, the tube 10 comprises a panel 12 having phosphors screen deposited on the inner surface thereof for emitting dotted light in three colors of blue, green and red and a funnel 14 integrally connected with the panel 12 thereby to form a vacuum envelop.
Also, an electron gun assembly 18 is provided in a neck portion 16 of the funnel 14 so as to emit electron beams arranged in line and consisting of a center beam and a pair of side beams that pass on the same horizontal plane. Three electron beams emitted from the electron gun assembly 18 are deflected by a horizontal and vertical magnetic fields generated by a deflection yoke 20 which is installed outside the funnel 14.
Further, a shadow mask 24 functioning as a color selection means is installed into the panel 12 to be spaced at a given distance from the inner surface of the panel 12, with being supported by a frame portion 22.
In such construction of the color cathode ray tube 10, the electron gun assembly 18 comprises a cathode 28 constituting a triode portion and a series of electrodes arranged orderly toward the cathode 28, as shown in FIG. 2.
More specifically, the electron gun assembly 18 comprises a control electrode 30 and an accelerating electrode 32 arranged orderly for focusing and accelerating electrons emitted from cathode 28 so as to form electron beams. These electron beams are then greatly focused by a prefocusing lens formed of the potential difference of the third grid electrode 44, the fourth grid electrode 46 and a focusing electrode 34 which are arranged orderly.
Also, an anode electrode 36 arranged next to the focusing electrode 34 forms a main lens that is formed by a potential difference applied between the focusing electrode 34 and the anode electrode 34. The electron beams passing through the main lens 42 are focused more sharply and then accelerated so as to form an electron beam spot, which will be then forwarded to the phosphor screen deposited on the inner surface of the panel 12.
Since the lens region of the accelerating electrode 32 with respect to the respective cathode 28 is more elongated in the horizontal direction than in the vertical direction due to the in-line arrangement of the respective cathode 28 as shown in FIG. 3, a shape of the electron beam spot is subject to a focusing deterioration in the peripheral portion of the panel 12 as shown in FIG. 4.
In order to prevent the focusing deterioration, according to Japanese Patent Application KOKAI Publication No. Sho 53-18866, as shown in FIG. 5, there is provided a groove, that is, a recess 48 elongated in the horizontal direction along the peripheral portion of the electron beam passing aperture (h) of the accelerating electrode 32 in the direction of the third grid electrode 44 so that the lens region in the horizontal direction with respect to the respective electron beam passing aperture (h) can be narrowed.
The recess 48 allows the depth of the electrode thickness direction to be large and thus the width of the electron beam spot shape before incidence upon the main lens to be elongated in the horizontal direction as shown in FIG. 6, thereby making the electron beams have a large astigmatism and preventing a deflection aberration.
Additionally explaining, an aspect ratio (b/a) in accordance with the horizontal elongation of the electron beam is closely related to the difference between the vertical direction thickness and the horizontal direction thickness of the electron beam passing aperture (h) in accordance with the depth of the recess 48 as shown in FIG. 7 or FIG. 8.
As shown in FIG. 6, the length (b) of the vertical direction width and the length (a) of the horizontal direction width of the electron beam are closely related to the diameter of the electron beam passing aperture (h) and the thickness of the fourth grid electrode 46, and the relationship between the aspect ratio (b/a) before incidence upon the main lens and its size affects the size of the electron beam spot on the overall region of the screen and thus the resolution and moirxc3xa9 as shown as an experimental value in FIG. 9.
Herein, moirxc3xa9 is a phenomenon that if the electron beam spot diameter bercomes smaller than a value determined by a periodic structure of phosphor dots, the periodic structure of the phosphor dots and electron beam scanning lines (or periodic video signal) are interfered with each other to thereby cause a stripe pattern 50 on the screen as shown in FIG. 10. This moirxc3xa9 phenomenon also occurs in such a manner that the horizontal direction of the screen is inclined toward the center direction to thereby cause a distortion of the screen (this is referred to as a video moirxc3xa9).
Again, with respect to the above-described Japanese Patent Application KOKAI Publication No. Sho 53-18866, this reference uses only a recess 48 having a large depth to adjust the aspect ratio of the electron beam before incidence upon the main lens to thereby reduce the deflection aberration and thus to prevent a deterioration of the electron beam spot on the center of the screen as can be seen from its invention spirit. However; this may cause a large blooming of the vertical direction diameter of the electron beam spot in the center of the screen due to astigmatism and thus a video moirxc3xa9on the overall region of the screen.
As a result, such a technique for controlling the lens region of the respective electron beams by forming the recess 48 on the accelerating electrode 32 has a problem that cannot solve a moirxc3xa9 phenomenon as well as deterioration due to the electron beam spot.
Accordingly, the present invention has been made in order to solve the above problem, and an object of providing an electron gun assembly for a color cathode ray tube capable of preventing a deterioration of focus characteristics of the electron beams and a moirxc3xa9 phenomenon thereby realizing a high quality image.
In order to achieve the above object, in an electron gun assembly for a color cathode ray tube according to an embodiment of the present invention comprising a series of orderly arranged electrodes including a control electrode, an accelerating electrode, the third grid electrode and the fourth, plate-shaped grid electrode having a predetermined thickness in order to form, focus and accelerate the electron beams by inducing electrons emitted from cathodes constituting a triode portion to pass electron beam passing apertures, said accelerating electrode is provided with a recess which has a both-side peripheral portion width B in its horizontal direction elongated, compared to its vertical direction width W with respect to the electron beam passing aperture having a predetermined diameter A and which has a predetermined depth D in the third grid electrode direction, and a shape of the accelerating electrode and the fourth grid electrode satisfies the following equation:
2.8 mmxe2x89xa6D/W+T/Axe2x89xa63.2 mm